{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2026,9,30]],"date-time":"2026-09-30T21:10:53Z","timestamp":1790802653035,"version":"4.1.0"},"reference-count":38,"publisher":"Frontiers Media SA","license":[{"start":{"date-parts":[[2026,8,4]],"date-time":"2026-08-04T00:00:00Z","timestamp":1785801600000},"content-version":"vor","delay-in-days":0,"URL":"https:\/\/creativecommons.org\/licenses\/by\/4.0\/"}],"content-domain":{"domain":["frontiersin.org"],"crossmark-restriction":true},"short-container-title":["Front. Bioeng. Biotechnol."],"abstract":"<jats:sec>\n                    <jats:title>Introduction<\/jats:title>\n                    <jats:p>\n                      Natural synovial knee joints exhibit very low friction, enabling efficient long-term function. In contrast, total knee arthroplasty (TKA) introduces artificial articulating surfaces with altered kinematic and tribological conditions. However, how\n                      <jats:italic>in vivo<\/jats:italic>\n                      friction evolves across different dynamic activities after TKA remains unclear. The aim of this study was to quantify\n                      <jats:italic>in vivo<\/jats:italic>\n                      knee joint friction after TKA and to investigate its relationship with activity-dependent loading conditions and knee joint kinematics.\n                    <\/jats:p>\n                  <\/jats:sec>\n                  <jats:sec>\n                    <jats:title>Methods<\/jats:title>\n                    <jats:p>\n                      <jats:italic>In vivo<\/jats:italic>\n                      joint friction was assessed in three subjects with instrumented telemetric knee implants during chair sit, squat, and level walking. Joint contact forces and moments were recorded\n                      <jats:italic>in vivo<\/jats:italic>\n                      , while synchronous fluoroscopic imaging was used to determine knee joint kinematics. Loading data were referenced to the instantaneous axis of rotation to calculate frame-by-frame knee joint friction throughout each activity.\n                    <\/jats:p>\n                  <\/jats:sec>\n                  <jats:sec>\n                    <jats:title>Results<\/jats:title>\n                    <jats:p>\n                      <jats:italic>In vivo<\/jats:italic>\n                      friction after TKA was highly dynamic and activity-dependent, showing distinct temporal variations across the three different activities. During chair sit and squat, coefficients of friction (\u00b5) of up to approximately 0.15 were observed during loaded flexion and extension phases, consistent with mixed or boundary lubrication regimes reported\n                      <jats:italic>in vitro<\/jats:italic>\n                      . In contrast, substantially higher \u00b5 values of up to 0.3 during chair sit and 0.43 during gait were observed during low-loading or rapid kinematic transition phases. Peak friction moments reached up to 5.5\u00a0Nm during chair sit, 5.0\u00a0Nm during squat, and 8.9\u00a0Nm during gait. Overall, friction behavior varied substantially throughout the movement cycle and exhibited pronounced phase-dependent and inter-individual variability with peaks during swing phases.\n                    <\/jats:p>\n                  <\/jats:sec>\n                  <jats:sec>\n                    <jats:title>Discussion<\/jats:title>\n                    <jats:p>\n                      These findings demonstrate that\n                      <jats:italic>in vivo<\/jats:italic>\n                      knee joint friction after TKA is dynamic, activity- and phase-dependent. The observed friction dynamics illustrate that friction is only partially linked to contact load magnitude, while motion velocity and possible lubrication-related effects may also influence friction. These findings may help improve experimental and computational models used in TKA tribology and implant evaluation.\n                    <\/jats:p>\n                  <\/jats:sec>","DOI":"10.3389\/fbioe.2026.1885711","type":"journal-article","created":{"date-parts":[[2026,8,4]],"date-time":"2026-08-04T05:38:28Z","timestamp":1785821908000},"update-policy":"https:\/\/doi.org\/10.3389\/crossmark-policy","source":"Crossref","is-referenced-by-count":1,"title":["In vivo knee joint friction after total knee arthroplasty is highly dynamic and phase-dependent"],"prefix":"10.3389","volume":"14","author":[{"given":"Julian N.","family":"Zierke","sequence":"first","affiliation":[{"name":"Julius Wolff Institute, Berlin Institute of Health at Charit\u00e9 \u2013 Universit\u00e4tsmedizin Berlin","place":["Berlin, Germany"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Philippe","family":"Moewis","sequence":"additional","affiliation":[{"name":"Julius Wolff Institute, Berlin Institute of Health at Charit\u00e9 \u2013 Universit\u00e4tsmedizin Berlin","place":["Berlin, Germany"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Rainald M.","family":"Ehrig","sequence":"additional","affiliation":[{"name":"Julius Wolff Institute, Berlin Institute of Health at Charit\u00e9 \u2013 Universit\u00e4tsmedizin Berlin","place":["Berlin, Germany"]},{"name":"Zuse Institute Berlin (ZIB)","place":["Berlin, Germany"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"William R.","family":"Taylor","sequence":"additional","affiliation":[{"name":"Institute for Biomechanics, ETH Z\u00fcrich","place":["Z\u00fcrich, Switzerland"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Adam","family":"Trepczynski","sequence":"additional","affiliation":[{"name":"Julius Wolff Institute, Berlin Institute of Health at Charit\u00e9 \u2013 Universit\u00e4tsmedizin Berlin","place":["Berlin, Germany"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Georg N.","family":"Duda","sequence":"additional","affiliation":[{"name":"Julius Wolff Institute, Berlin Institute of Health at Charit\u00e9 \u2013 Universit\u00e4tsmedizin Berlin","place":["Berlin, Germany"]}],"role":[{"vocabulary":"crossref","role":"author"}]},{"given":"Philipp","family":"Damm","sequence":"additional","affiliation":[{"name":"Julius Wolff Institute, Berlin Institute of Health at Charit\u00e9 \u2013 Universit\u00e4tsmedizin Berlin","place":["Berlin, Germany"]}],"role":[{"vocabulary":"crossref","role":"author"}]}],"member":"1965","published-online":{"date-parts":[[2026,8,4]]},"reference":[{"key":"B1","doi-asserted-by":"publisher","first-page":"357","DOI":"10.1002\/art.1780100407","article-title":"Hyaluronic acid in synovial fluid. 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